1. Field of the Invention
The present invention relates to an insulation and a fabrication method thereof for a composite pressure vessel having an improved adhesiveness and particularly to an insulation and a fabrication method thereof for a composite pressure vessel which is capable of tolerating relatively high pressure and protecting the pressure vessel from high temperature by providing better adhesiveness between the insulation and the pressure vessel.
2. Description of the Conventional Art
The filament winding method of fabricating a typical pressure vessel is classified into a wet winding method and a dry winding method in accordance with materials used therein.
Of which, the wet winding method is directed to inserting an epoxy resin into a fiber while a carbon fiber in a roving form passes through an epoxy resin tube and to forming a carbon fiber in which an epoxy resin is formed on the surface of the insulation rubber (EPDM: Ethylene Propylene Diene Monomer) provided at an outer portion of a sand mold.
The dry winding method is directed to directly forming a prepreg roving in which an epoxy resin is formed in a carbon fiber in a roving form on an outer portion of a sand mold.
According to the above-mentioned dry winding method, since the viscosity of an epoxy resin used therein is high, the epoxy resin does not accumulate at one side during the curing process therefore rotating a mandrel is unnecessary, and an autoclave (vacuum and formation pressure: 10 kg/cm.sup.2) can be used to cure the epoxy resin. Therefore, it is possible to achieve better adhesiveness between the insulation and the pressure vessel when the same adhesive employed in the wet winding method is used. However, the dry winding method has disadvantages in that the materials used therein are expensive.
Meanwhile, the wet winding method has the advantage that the materials used therein are not expensive compared with the dry winding method. However, in order to prevent the epoxy resin from being accumulated on one side it must be rotated in the oven because the epoxy resin has a relatively low viscosity. Since insulation rubber, which is widely used as an insulation material, has non-polar characteristics, the insulation rubber does not possess better adhesiveness with respect to the adhesive used therein. So as to overcome these disadvantages, a primary process with respect to the surface of the insulation rubber was introduced; however, the problems were not improved substantially. Another method was introduced so as to improve the problems. That is, so as to improve adhesiveness between the pressure vessel and the insulation, an epoxy resin is forcibly penetrated to the surface of the insulation during a pressure vessel fabrication. In addition, so as to increase the surface between the epoxy resins constituting the insulation and the pressure vessel, the following method was introduced in the industry. That is, there are certain pressures in the insulation, of which one is a pressing force which occurs when forming a carbon fiber on the rubber surface of the mandrel, and of which another one is applied between the outer surface of the insulation and a composite pressure vessel because an internal mandrel and the insulation have a certain thermal expansion coefficient higher than that of the carbon fiber at the room temperature of 25.degree. C. after a wet winding process is completed.
Meanwhile, carbon fiber, which is weak with respect to external impact, can be broken due to pressure accumulated in the insulation. Therefore, reducing the force applied to the carbon fiber is necessary, compared with when a glass fiber receives an external force. In addition, the pressure between certain elements, which occurs due to the different thermal expansion coefficients in the wet winding process, is smaller than that of the pressure applied in the autoclave process of the dry winding method, and there is a limit in applying a vacuum to the boundary portion between certain elements to remove air gaps. Therefore, the wet winding method using carbon fiber has relatively low adhesiveness compared with the dry winding method.
In addition, stress between the pressure vessel and the insulation exists due to the different thermal expansion coefficients which occur due to a temperature decrease from a curing temperature of 150.degree. C. to room temperature of 25.degree. C. Generally, as the temperature (minimum level of -40.degree. C.) is decreased, and the rubber grows thicker, the residual stress grows greater. Therefore, the non-contact surface at the boundary portions between corresponding elements disadvantageously increases.
In addition, the price of the materials is about twice the price in case that a prepreg roving state exists in the epoxy resin containing a carbon fiber compared that a carbon fiber is contained in the epoxy resin during a winding process. That is, the materials used on the dry winding method is expensive compared with the wet winding method.